Final answer:
Hess's law is valuable for calculating the enthalpy changes of reactions when it is impractical or inconvenient to measure them directly, allowing for the use of standard enthalpy values and thermochemical cycles to determine heat of reaction.
Step-by-step explanation:
Hess's law is invaluable for collecting thermodynamic data because it offers a way to calculate the enthalpy change of a reaction without direct experimental measurement. It is particularly useful when reactions are difficult or impossible to measure or when it's more convenient to determine the heat of reaction through calculations. For example, the law allows us to use a thermochemical cycle, such as the Born-Haber cycle, to find lattice energy or standard enthalpies of formation when they are challenging to determine experimentally.
By understanding Hess's law, we can ascertain the enthalpy change by addition if a process can be broken down into multiple, measurable steps. This method simplifies the collection of thermodynamic data since we can rely on a standard set of tabulated enthalpy values, starting from the elemental forms. Ultimately, Hess's law facilitates the calculation of enthalpy changes for a broad range of chemical reactions efficiently and accurately, making it a powerful tool for chemists.